1. Technical Field of the Invention
This invention relates to a surface-roughened film and methods for fabricating such a film. The surface-roughened film has good transparency/exceptional clarity and high lubricity so that it is useful for a photoengraving material, a label, a packaging film, a magnetic recording film, and a clear film.
2. Description of the Prior Art
A polyester film represented by a polyethylene terephthalate film exhibits exceptional physical and chemical properties, so that it can be applied for various products such as a magnetic recording material, a wrapping material, an electrical material, a sensitized material, and many photographic materials.
For example, it is well known that the polyester films dominate the market share in the magnetic tape industries and the lubricity (slip characteristics) of the film significantly affects the manufacturing process or succeeding processing of the films and, in turn, affects the quality of intermediate or end products.
In order to obtain this lubricity, particulate inorganic compounds are generally incorporated into the polymer to generate irregularity on the surface of the film by means of drawing. Such a technique is described in, for example, the Japanese Patent Unexamined Publication No.63-137927. However, in such a process it is difficult to uniformly disperse the fine particles because the particulate inorganic compounds tend to aggregate or agglomerate with each other. With respect to this problem, Japanese Patent Unexamined Publication No.63-66222 describes a method in which the particulate inorganic compounds are added to the polymer on its polymerization.
Recently, transparent films are in great demand. Higher transparency is required for polyester film, which is so called "thick-walled film", having a thickness in the range of 50 to 200 .mu.m which is useful for a microfilm, an electrophotograph, an X-ray photograph, a diazo print, a slidefilm (filmstrip) for an overhead projector (OHP), a label, and a laminated film. Generally, a thick film absorbs and scatters more light compared with a thin film, so that the transparency is reduced though thick-walled film having high transparency has also been commercially available.
However, as described below, such films are not easy to handle. More particularly, films are generally fabricated in various steps such as drawing, rolling, slitting and working before use as end products. It is indispensable to prevent blocking of the film and to add the lubricity to the film for facilitating handling thereof when passing through these steps. Troublesome handling in these steps results in defective end products. In order to facilitate handling, the film is usually incorporated with particulate inert materials of inorganic compounds to cause adequate irregularity on the film surface and thereby prevent blocking of the films. In addition, incorporation of such particles contributes to the proper lubricity between films and, in turn, prevents trouble accompanying rolling-up of the films used as intermediates or end products. The above-mentioned lubricity of the film is improved with reduction of processing problems by increasing the volume of the added particles, but the increased amount of particulate additive is disadvantageous in that it lowers the transparency of the film and causes some loss of the desired optical characteristics and visual appearance of the film.
On the other hand, for increasing the desired transparency of the film it is necessary to reduce the volume of the added particulate inert materials of the inorganic compound, resulting in lower lubricity of the film and causing troubles in handling. One example of such handling trouble caused by the lower lubricity is that small protuberances are generated on the film surface of a rolled film upon rolling up the films after drawing. An excessive amount of such protuberances may cause not only bad appearance of the film but also considerable damage to the film used in industries where precision and accuracy of the film transparency are required for optical, photographic, or graphical purposes. Thus, it is difficult to satisfy both transparency and lubricity, which are antipodal properties.
In addition, in the above-mentioned method where the particulate inert materials of an inorganic compound are added, the productivity of the film is exceedingly low because of clogging of a screen with the inorganic particles in filtering during melt film formation. While a screen having a larger mesh size may allow some foreign matter to be incorporated into the film, this is disadvantageous in that such a film is often broken upon drawing. Further, even a small amount of inorganic particles may cause wearing of the T-die lip or T-die blade used in melt film formation over a long period. Accordingly, a solution to the above-mentioned problem has long been desired. The incorporated particles also affect lifetime of a knife used in a slitting process or other cutting process of the film, that is, an exceedingly shortwearing knife will necessitate frequent replacement of the knife, resulting in reduced productivity. In addition, the film with some additive or foreign matters is disadvantageous when being used for a magnetic tape or other magnetic recording media, because a head of a recorder may be worn by the incorporated particles.
Conventionally, most solutions proposed for the above-mentioned "thick-walled film" compromise between transparency and lubricity of the film rather than attempting to improve both together. For example, the compromise solutions include highly controlled processing conditions with expensive plant investment and reduction of processing speeds or take-up speeds.
However, it would be highly desirable for a thick-walled transparent film to have both improved transparency and lubricity without improvement in the fabricating device in view of the necessity for increasing processing speeds and improving productivity.